The present application is related to the head design for an optical data storage system.
The term "optical" as used herein is intended to apply to any directed energy beam system including, for example, visible light, infrared radiation or electron beam.
Realizing the full potential of high density optical storage depends on optimal head design. Optical storage systems such as magneto-optical (MO), write once read many (WORM) and compact disk (CD ROM) all depend on precisely positioning an optical head to train a laser beam on a nominally selected track. The optics, laser transmitter/detector, auto focusing and fine tracking mechanisms are carried on a linear or rotary actuator. In MO data storage systems, the actuator, typically an electromagnetic mechanism, positions an optical head over the storage track portion of an MO disk. The head comprises the laser diode, optics, detectors, focus and fine tracking actuators and servo electronics, which collectively perform the reading and writing operations. Typically, writing onto an MO disk requires a two-pass operation in which the track or sector on which data is to be written is first erased. In the erase cycle, an externally applied magnetic field is established in a direction normal to the disk. The external field is typically provided by energizing a fixed electromagnetic coil disposed on the opposite side of the disk from the objective lens. The coil is usually elongated to cover the stroke of the head. Laser-heated domains to be overwritten are returned to a single orientation under the influence of the external field. After the erase cycle, the external field is reversed and the laser beam is pulsed to write data by selectively reversing the polarity of domains where necessary. The erase cycle significantly reduces disk drive performance by adding one full latency cycle to the writing process.
In present optical storage devices, focusing and fine tracking operations are often accomplished by a 2-degree-of-freedom, miniature voice coil actuator located in the optical head. In this method, which requires a closed-loop servo system, a portion of the light reflected off of the disk is directed onto an astigmatic sensor. Focus and track error signals generated by the sensor are fed back as control signals to the voice coil actuator. The focus and fine-track actuator mechanisms encumber the integrated head with compliant components which limit its tolerance for high accelerations accompanying fast seeks. A single actuator performing both functions also instigates cross coupling.
In addition, the mass of present optical heads, due largely to the weight of the objective lens, limits acceleration sensitive access times.